This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2001-73844, filed on Mar. 15, 2001; the entire contents of which are incorporated herein by reference.
This invention relates to a photo-detecting semiconductor device and an electronic apparatus having such a photo-detecting semiconductor device.
Optical communication using optical signals is often utilized at internal and/or external portions of an electronic device. In optical communication, an optical signal taken out of an optical transmission device is received at a photo-detecting device via a transmission path such as optical fibers, for example. The photo-detecting device outputs an electric signal responsive to the optical signal, and the electric signal is used to control the electronic device or for other purposes.
Configuration and behaviors of a conventional photo-detecting semiconductor device are explained below with reference to FIG. 5. FIG. 5 illustrates an internal circuit 300 of a conventional photo-detecting semiconductor device. The internal circuit 300 includes a photodetector element 315 that receives an optical signal from an optical transmission path and generates an electric signal responsive to the optical signal; an amplifier circuit 365 that amplifies the electric signal generated by the photodetector element 315; a discriminator circuit 390 supplied with power from a power source to discriminate the electric signal amplified by the amplifier circuit 365; a power supply circuit 350 that supplies electric power in response to the discrimination result of the discriminator circuit 390; and a shaping/output circuit 370 supplied with power from the power supply circuit 350 to shape the waveform of the electric signal amplified by the amplifier circuit 360 and output it. The amplifier circuit 365 includes a changeover mechanism that changes the consumed current level to save the consumed current unless a signal is input. The changeover mechanism limits the electric current consumed by the amplifier circuit 365 to the level supplied from the power source and decreases the response speed of the amplifier circuit 365 in the waiting state where there is no input signal and no power is supplied from the power supply circuit 350. As a result, the current supplied by the amplifier circuit 365 is limited to approximately one half the current in its operation state. Similarly, the discriminator circuit 365, in its standby mode, is not supplied with electric power from the power supply circuit 350, and the current consumed thereby is limited to the quantity supplied from the power source. Therefore, its current consumption is reduced as compared with that in its operation mode.
Here is explained an operation upon entrance of an optical signal into the photo-detecting device or electric apparatus. In the photo-detecting device heretofore held in the standby mode, the photodetector element 315 receives the optical signal, and generates a corresponding electric signal. The amplifier circuit 365 amplifies this electric signal inputs and supplies amplified signals to the discriminator circuit 390. The discriminator circuit 390 compares the voltage based on the electric signal from the amplifier circuit 365 with a reference voltage, and when the electric signal exceeds the predetermined voltage, it outputs to the power supply circuit 350 an electric signal for starting the supply of an electric power. In receipt of the electric signal, the power supply circuit 350 supplies power to the discriminator circuit 390 and the shaping/output circuit 370. The electric signal is amplified by the amplifier circuit 365 having supplied with power from the power supply circuit 350 and currently having its true responsive speed. Then the amplified signal is shaped in waveform by the shaping/output circuit 370, and output as an output electric signal from the photo-detecting device. The output-use electric signal output from the photo-detecting device is used for controlling an electric apparatus, for example.
Next explained is an operation upon no input of optical signal into the photo-detecting device or electric apparatus from the optical transmission path. Since the photodetector element 315 generates no electric signal, the discriminator circuit 390 generates no detection electric signal. Therefore, the power supply circuit 350 supplies power to none of the amplifier circuit 365, discriminator circuit 390 and shaping/output circuit 370. As a result, the mode enters in the standby mode where no output signal is output and the power consumption is small. In the operation mode, both the power from the power supply circuit 350 and the power direct from the power source are supplied to the amplifier circuit 365 and the discriminator circuit 390. In the standby mode, however, the power from the power supply circuit 350 is not supplied, and the power direct from the power source alone is supplied to the amplifier circuit 365 and the discriminator circuit 390. Therefore, in the standby mode, the amplifier circuit 365 and the discriminator circuit 390 receive power less than that in the operation mode directly from the power source, and carry out detection of an optical signal or no optical signal under a condition with a lower response speed.
Circuits used in electric apparatuses have recently been desired to deal with signals at progressively higher speeds. In order to process high speed signals with a high capability, in general, circuit elements have to be operated with the maximum performance. For this purpose, the amplifier circuit needs more electric current. The shaping/output circuit 370 for outputting the output-use electric signal needs a large quantity of current for driving the transmission path connecting to the output at a high speed. On the other hand, since the issue concerning energy strongly presses for decreasing power consumption, it is necessary to minimize the power consumption of the photo-detecting semiconductor device and the electric apparatus in the no-signal mode, i.e. in the standby mode.
In the conventional photo-detecting semiconductor device and the electric apparatus, the power supply circuit 350 interrupts the supply of power to the shaping/output circuit 370, amplifier circuit 365 and discriminator circuit 390 in the standby mode, and the power source directly supplies power to the amplifier circuit 365 and the discriminator circuit 390. In this manner, the conventional technique decreased the supply of power to the photo-detecting semiconductor device and the electric apparatus to minimize the power consumption in the standby mode.
However, in regard to the amplifier circuit 365, for example, since its circuit design must meet its true performance in the high-response mode, i.e. in the operation mode, the supplied power in the standby mode cannot be decreased so much. Additionally, to have the amplifier circuit 365 operate with less supplied power in the standby mode than in the operation mode, it is necessary to have a single amplifier cope with two different modes, namely, the standby mode and the operation mode. This requirement inevitably complicates the amplifier circuit 365. Moreover, complication of the circuit encumbered full performance of the true capability of the circuit element.
According to an embodiment of the present invention, there is provided a photo-detecting semiconductor device comprising: a first photodetector element and a second photodetector element placed adjacent to each other and isolated from each other to each generate an electric signal responsive to a received optical signal;
a signal detecting circuit which amplifies the electric signal from said first photodetector element and outputs a detection-use electric signal to detect a predetermined optical signal received at said first photodetector element;
a power supply circuit which starts the supply of power upon receiving said detection-use electric signal from said signal detecting circuit; and
a signal output circuit supplied with power from said power supply circuit to amplify the electric signal from said second photodetector element and output an output-use electric signal.
According to another embodiment of the present invention, there is provided an electric apparatus having a photo-detecting semiconductor device to start operation upon receipt of a detection-use electric signal or an output-use electric signal from said photo-detecting semiconductor device, said photo-detecting semiconductor device comprising:
a first photodetector element and a second photodetector element placed adjacent to each other and isolated from each other to each generate an electric signal responsive to a received optical signal;
a signal detecting circuit which amplifies the electric signal from said first photodetector element and outputs said detection-use electric signal for detecting a predetermined optical signal received at said first photodetector element;
a power supply circuit which starts the supply of power upon receiving said detection-use electric signal from said signal detecting circuit; and
a signal output circuit supplied with power from said power supply circuit to amplify the electric signal from said second photodetector element and output said output-use electric signal.